1. Field of the Invention
The present invention relates to a flat panel display and a method of manufacturing the same, and more particularly, to a flat panel display that is easy to manufacture while providing improved brightness and external light coupling efficiency, and a method of manufacturing the same.
2. Description of the Related Technology
The external light coupling efficiency ηex of a flat panel display can be expressed as Equation 1 below.ηex=ηin·ηout  (Eq. 1)
where ηin and ηout are an internal light coupling efficiency and an output coupling efficiency, respectively
The internal light coupling efficiency ηin is determined by self-destructive light in respective layers of the flat panel display, and the output coupling efficiency ηout is determined by a total reflection between the respective layers. The total reflection is a phenomenon that light is totally reflected when it advances from a high refractive layer to a low refractive layer at angles above a critical incidence angle.
In the case of the flat panel display, light emitted from an emission layer passes through many layers until it is outputted to an outside of the display. Therefore, there may exist light that cannot be outputted to the outside due to the refractive index of each layer.
In Equation 1, when considering the total reflection between the respective layers, a light transmission efficiency ηout of light emitted from the emission layer to the outside can be expressed as Equation 2 below.ηout=(½)(Nout/Nin)2  (Eq. 2)
where N is the refractive index of each layer
When light travels from a layer with a refractive index of about 1.5 to a layer with a refractive index of about 1.2, the light transmission efficiency can be calculated at 32% from Equation 2. That is, it can be seen that about 70% of light incident on the interface between the two layers is lost while traveling multiple layers.
Many attempts have been made to prevent the degradation of the external light coupling efficiency.
An example of the above attempts is to increase a supply voltage so as to improve the brightness. This method, however, requires a large capacity of battery and thus conflicts with lightweightness. The method also shortens the lifetime of the battery and pixels in a display. Therefore, there have been proposed the following prior arts for improving the brightness while reducing the supply voltage.
Japanese Patent Laid-open Publication No. HEI 4-192290 discloses an inorganic electroluminescence (EL) apparatus in which inorganic EL devices are formed in a transparent substrate and a plurality of condensing microlenses whose size is equal to or larger than the size of the inorganic EL device are formed on an outer surface of the transparent substrate. Light is incident on the interface between the transparent substrate and air at angles above a critical angle, and its incident angle is reduced below a critical angle in the microlens, thereby reducing the total reflection. Also, the light is outputted in a given direction, thereby improving the brightness in the given direction. However, since the EL device is a surface light source, it inevitably generates undesirable EL light that is diffused rather than condensed. Also, images from the neighboring EL devices overlap with each other, thereby degrading the image sharpness.
Japanese Patent Laid-open Publication No. HEI 7-037688 discloses an EL device that is formed in a substrate including a high refractive portion that is formed of a higher refractive material than its peripheral portion. Light emitted from the EL device is outputted through the high refractive portion, thereby improving the external light coupling efficiency. EL light having passed through the high refractive portion is diffused as illustrated in FIG. 1 of the Publication. Therefore, the front brightness cannot be improved greatly.
Japanese Patent Laid-open Publication No. HEI 10-172756 discloses an organic EL apparatus in which one or more condensing lenses are formed between a lower electrode constituting an organic EL device and an outer surface of a transparent substrate and the organic EL devices are provided corresponding to the condensing lens. Light, which was emitted from the organic EL and having passed through the condensing lens, is incident on the interface between the transparent substrate and air at angles below a critical angle, thereby improving the external light coupling efficiency. However, images from the neighboring EL devices overlap with each other, thereby degrading the image sharpness.